The present invention relates to an output level control circuit for a radio-frequency transmitter, and more particularly to an output level control circuit for a radio-frequency transmitter which intermittently transmits a radio-frequency signal such as a transmitter in a digital cellular mobile telephone system or the like.
An output level control circuit which maintains the transmit power of a radio-frequency transmitter at a predetermined output level comprises a radio-frequency signal detector (referred to as a signal detector hereinafter) which detects the peak level of a radio-frequency signal output from a variable gain amplifier unit. The control circuit also comprises a comparator unit which compares the output of the signal detector and a reference voltage corresponding to the predetermined output level, and a control unit which controls the output level of the variable gain amplifier unit in response to the output of the comparator unit.
In order to maintain the amplifier's output at a constant level over a wide range of temperature, the signal detector is required to be able to detect with high accuracy the radio-frequency power level over a wide range of temperature. A signal detector of this type is disclosed in U.S. Pat. No. 4,523,155 (issued on Jun. 11, 1985), which detector is aimed at satisfying the above-mentioned requirement. The disclosed signal detector includes a detecting diode which produces a detection output in response to the envelope of the radio-frequency signal, and a temperature compensating diode which has characteristics substantially the same as those of the detecting diode and is thermally connected to the detecting diode. In this signal detector, a forward bias voltage is supplied to the detecting diode via a bias voltage setting resistor from a bias voltage supply source. The detected signal is obtained as the superposition of the detected voltage and the bias voltage via a load resistor connected to the detecting diode. The forward bias voltage is also supplied to the temperature compensating diode from the bias voltage supply source, wherein the voltage of the bias voltage supply source is changed accompanying the variations in the forward voltage of the temperature compensating diode. Since the bias voltage of the detecting diode is the result of subtracting the forward voltage of the detecting diode from the voltage of the bias voltage supply source, it will be canceled by the temperature variations of the forward voltage of the temperature compensating diode even if it contains temperature variations. Accordingly, this signal detector is capable of producing a detected signal which is not affected by the changes in the temperature.
The comparator unit compares the reference voltage that corresponds to a predetermined output level of an intermittent transmitting signal with the voltage of the detected signal, and the comparison output is supplied to the control unit. The control unit controls the gain of the variable gain amplifier unit so as to bring the detected signal to conform with the reference voltage. When the reference voltage is raised or lowered, the detected signal, which corresponds to the output level of the transmitting signal, has to quickly follow the raised or lowered reference voltage. For example, in accordance with the recommended standard (EIA Interim Standard, IS-19-B, 3.1.3.3, January 1988, U.S.A.) for the 800 MHz cellular subscriber units of the EIA system, it is prescribed that the rise and the fall times of the transmitting signal be less than 2 milliseconds.
Since, however, the above-mentioned output level control circuit carries out the control of the output level, at the time of the rise of the transmitting signal, only when the reference voltage is higher than the signal detector output, it is not possible to perform the control of the output level until the reference voltage becomes higher than the bias voltage of the detecting diode when a bias voltage is applied to the detecting diode for the purpose of temperature compensation as mentioned above. Consequently, at the time of starting transmitting radio power, the output voltage from the detecting diode does not change until the reference voltage exceeds the bias voltage, resulting in a problem that the rise of the radio transmitting power is delayed by that period, being unable to satisfy the above-mentioned standard rise time.
A remedy is available to hasten the rise of the transmitting power by giving an extremely high value to the control loop gain of the output level control circuit. However, the adoption of this remedy not only makes the circuit tend to generate the overshoot of the transmitting power or the oscillation of the control loop, but also requires the addition of parts for amplification and filtering to the control loop.